An evaporative control system for a multicylinder internal combustion engine, in general, has a purge passage that connects a canister for temporarily storing evaporated fuel from a fuel tank and an intake air passage of an internal combustion engine (hereinafter simply referred to as an engine) and a purge control valve disposed in the intake air passage. The purge control valve is activated depending on the engine operating condition, and is opened and closed at a given frequency in response to a pulse signal having a given duty factor. If a period of the pulse signal is substantially synchronized with an engine cycle, fuel purged from the canister will be drawn into a specific cylinder of the engine to make an air-fuel ratio in the cylinder rich and an air-fuel ratio in any other cylinder that draws no purged fuel lean, thereby fluctuating an air-fuel ratio in the engine as a whole and causing misfires in the lean cylinders. To solve this problem, a technique to change the driving frequency of the purge control valve to another driving frequency when the driving frequency is substantially synchronized with an engine cycle is disclosed (Refer to Japanese Patent Publication No. 6-241129).
However, the technique disclosed in Japanese Patent Publication No. 6-241129 suddenly changes the driving frequency of the purge control valve at a boundary where it is substantially synchronized with an engine cycle. This results in fluctuating an air-fuel ratio in the engine, and the exhaust emission is deteriorated. To converge the fluctuating air-fuel ratio to a target air-fuel ratio, the disclosure corrects a fuel injection quantity. It takes time, however, until the fluctuation is stabilized at the target air-fuel ratio.